As tin-silver alloy plating baths used for forming a tin-silver alloy plating film on a metal substrate or the like, an alkaline cyanide bath has been known.
However, since the bath contains the poisonous cyanide, the bath has problems that the bath per se has an extremely high toxicity which necessitates a special care in the handling and a special treatment of the waste water and that the working environment is polluted.
On the other hand, known non-cyanide acid baths, for example, include an alkanesulfonic acid or alkanolsulfonic acid bath, and mercaptoalkanecarboxylic acid and/or mercaptoalkanesulfonic acid as disclosed in Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") Nos. Hei 7-252684 and Hei 8-13185.
However, in such an acid bath, it was impossible to form a stable solution by dissolving silver which is an indispensable component for forming the tin-silver alloy together with tin. Namely, silver was insolubilized to form a black or brown precipitate immediately after or within 24 hours after the preparation of the plating bath. It was thus difficult to maintain the concentration of the components in the bath.
After intensive investigations, the present applicant had developed an acid bath which was an alkanesulfonic acid and/or sulfamic acid bath containing a thioamide compound and a thiol compound. Although silver is not precipitated or insolubilized immediately after the preparation of the plating bath or in 24 hours thereafter, the plating bath has a problem that the thioamide compounds and the thiol compounds such as mercaptosuccinic acid and mercaptolactic acid exert a harmful influence on the plating properties when the plating bath is left to stand or kept being used for a long period of time.
When the acid bath is left to stand at a high temperature, silver is insolubilized to form the black or brown precipitate within 24 hours like in the acid baths disclosed in the above-described J. P. KOKAI Nos. Hei 7-252684 and 8-13185 and thus, it is difficult to keep the concentration of each component in the bath.